Montségur stands on a rocky « pog » 1207 meters high (about 3620 feet) . The only way to reach the castle is a one hour walk on a steeply lane. The view while up there is worth climbing though.
Click to view attachment
Montsegur taken from the road below

The site shows archaeological signs of occupation as soon as Neolithic period. It was obviously occupied during the Roman Empire too.
Anyhow, prior to 1204, nothing much is known about the site, except there was a castle or a castrum (fortified village) up there at this time.

In 1204 anyhow, it was rebuilt by Raymond de Pereire and Pierre de Mirepoix, to become a shelter for the persecuted Cathars. After the defeat and submission of the count of Toulouse against the crusade army in 1229, Montségur became the head of the Catharian church and the heart of the southeast rebellion.

In 1242, the royal army besieged Montségur. For eleven month, the French army stepped around the pog, while mountain people climbed food supplies . The French army counted about 7 000 to 10 000 soldiers, while the citadel garrison was 350, plus about 200 refugees. Finally, gascon mountain boys were used to bring and build, piece by piece, a giant catapult on the Eastern plateau.
By March 16 1243, the citadel surrenders.
Over 200 cathars will refuse to abjure their faith, and will be burned alive all together that same day by the walls of the citadel.
The castle itself was destroyed to the ground.

Click to view attachment
The view over the valley of Ariège. In the corner of the road, you can see the car park, where the lane to the castle starts

The castle that satnds nowadays was built by the late XIIIth –early XIVth century on the ruins of the cathar fortress by the king of France, to survey the Ariège valley. It remained used until the late XVIIth century, and was then abandoned.
The place has been partially restored, to avoid further degradation, but remain as very romantic medieval ruins.

MOre to see and to know (sorry but all in french !):
http://www.citaenet.com/montsegur/
http://lescathares.free.fr/chateau/montseg.html

and to last but not least, including a virtual visit (with the path all the way up !!) :
http://www.audar.com/montsegur/index.htm

French language sites are fine, we can Babel fish them!
I'm always inspired by these mountain top sites, they must have dug very very deep wells to get below the water table, and must have been very difficult to keep supplied, and easily beseiged.

well, for water supplies, you can visit a couple of tanks, dug in the rock and hidden below the fortress.

They were not that easy to besiege, as it needed an army to surround the mountain it is built on. There are lots of tracks into the mountain, only defenders know, and some of them, only trained men can use (and thus forbid).
More than 7 000 was not enough to the french army to prevent mountain men to pass through with supplies.
It took more than a year and a giant catapult to lead the small garrison to surrender.

Almost no place, as thick as the wall scan be, could resist a real siege.
The real tactical point was:
how many people do you need to defend the place (if 300 men can stop an army for several months, the place is good),
and how many communication roads can you survey (if the site shows a clear view to hundreds of miles around, and makes you a hundred miles detour to be avoided, it is good too).

That's why all these medieval fortresses were used untill the frontier with spain got deplaced far south. Some of them kept in use centuries after though, only for their tactical defense, in case of an invasion.

As for the size of a garrison, I read recently that ?Harlech in Wales only required about 60 men to prevent it being taken. Need to find that reference.

I've heard it was 40 but could be wrong.

Many times caves under the fortress/castle was widened or re-worked for better accessibility and then flooded for a larger water containment area.

Yes I've seen that too.